A natural history notebook and project of the Missouri Botanical Garden

Author: Leighton Reid

The best time to plant a tree was twenty years ago. The second best time is now.-Anonymous

Addendum: That is, unless the tree will grow just fine without your help or the tree doesn’t really belong there. In that case, the best time might be never.

Planting a tree is rejuvenating. It gets you outside, it’s good exercise, and it’s often good for the planet. Really, trees give us an awful lot and don’t ask for much in return. Among their many gifts are food, shade, animal habitat, building materials, erosion control, and fuel. Trees also filter our water and suck carbon out of the air. In cities, trees collect grit and grime that would otherwise coat our lungs.

But tree planting is not the same as restoration. Ecological restoration is the process of assisting the recovery of a damaged ecosystem. Trees are integral to many ecosystems, like forests…

Damaged ecosystems don’t recover overnight, but sometimes that’s all the time that they get. CCSD scientist Leighton Reid describes new research about tropical secondary forests in southern Costa Rica, including how long these young forests last, what’s at stake, and how we can keep them around longer.

Regrowing tropical forests on marginal farm lands is one of the main ways that humans can prevent runaway climate change. With ample moisture and long growing seasons, tropical trees often can grow quickly and pull large amounts of carbon out of the atmosphere, storing it in their wood and keeping it from trapping heat. At the same time, young forests provide habitat for plants and animals and improve water quality for humans, among many other benefits.

But even in a moist, tropical climate, trees don’t grow instantly. Typically, it takes many decades for a recovering forest to stock up all of the carbon that it can hold. And it can take even longer for some plants (like orchids) and animals (like antbirds) to return. If a forest starts to grow back, but then someone cuts it down again, these time-dependent benefits never accrue.

In other words, the hopes and expectations that many people have for young tropical forests depend on young tropical forests growing old. So do they? Our new study suggests not.

To find out how long secondary forests persist, I teamed up with Matthew Fagan, a landscape ecologist at the University of Maryland Baltimore County, and Rakan Zahawi, director of the Lyon Arboretum, as well as two students, James Lucas at Washington University and Joshua Slaughter at UMBC.

We studied a set of historical, aerial photos from southern Costa Rica, which covered the time period from 1947-2014. Previously, Zahawi and colleagues had classified which areas in each photo were forest and which areas were farms or other non-forest land uses. By comparing the maps they made for each year, we were able to see where and when new forests appeared and how long they remained as forest before they were converted to some other land use (mostly farms).

The young forests did not last long. Half of the new forests disappeared before they were 20-years old. And 85% were cut down before they were 54-years old. Larger forests and forests near rivers lasted longer.

First, the bad news. Twenty years is not even close to the amount of time it takes for a young forest to become as diverse as an old-growth forest. For example, vascular epiphytes like orchids and bromeliads take more than 100 years to fully recover in young forests.

Carbon storage will also take a hit. If forests elsewhere in Latin America are as ephemeral as forests in southern Costa Rica, then carbon stocking over the next thirty years may be reduced by an order of magnitude.

Ephemeral forests could just be a problem in Costa Rica, but another study shows that secondary forests in eastern Peru have even shorter lifespans. There, secondary forests are cleared at a rate of 3-23% per year. Compared to that, the 2-3% per year rate of loss in southern Costa Rica is considerably better. And that’s not a good thing. Clearly we need more research on secondary forest persistence from other places.

There is some good news, though. Even though many new forests were short-lived, the ones that survived were predictable. And if we can predict where new forests will survive, we should also be able to help them survive longer. Larger forests and forests close to rivers were cut down less often than small forests and forests far from rivers. This suggests that restoring large, riparian forests could be a smart investment.

Governments and other organizations can also help forests persist by creating incentives for long-term forest management, providing resources to enable long-term management, and ensuring that local people will be able to enjoy the benefits that old forests provide.

We hope that this work will lead to stronger restoration commitments. Right now, dozens of countries are setting big targets for forest restoration. For example, in 2012 Costa Rica committed to restore a million hectares of degraded land by 2020 (an area about one fifth the size of the country). There is a great opportunity for Costa Rica and other ambitious countries to plan for long-term forest restoration.

If we can begin to restore a million hectares of forest by 2020, why not plan to restore a million hectares of 100-year old forest by 2120?

A trail through secondary forest at the Las Cruces Biological Station in southern Costa Rica. Photo by J. Leighton Reid.

Sometimes farmlands quickly regrow tropical forests on their own, but other times they don’t. Dr. Karen Holl, a professor at the University of California Santa Cruz, gives some rules of thumb for when we can save money on tropical forest restoration by letting nature do the work, and when we may need to invest in tree planting.

Ambitious targets are being set to restore tropical forest because of their importance in storing carbon, regulating water cycles, conserving biodiversity, and supporting the wellbeing of people who live in tropical countries. For example, the 20 × 20 Initiative aims to restore 20 million hectares of tropical forest in Latin America by 2020. This represents an area slightly smaller than the country of Ecuador. One big question is: How are we going to restore forests at this scale with limited funds?

One of the cheapest ways to restore forest is to let nature do the work and leave forests to recover on their own. This works in some sites where forests regenerate quickly. In other cases, usually sites that have been used intensively for agriculture, the land may be covered by tall grasses (up to 3 meters, or 10 feet high) for years. Our past research shows that even within a small region, the rate of natural forest recovery varies greatly.

Natural forest recovery is highly variable in southern Costa Rica, even after a decade of recovery. Left: slow recovery on a former farm, still dominated by non-native grasses, with an open canopy and little tree recruitment. Right: speedy recovery on a former farm, with virtually no grass cover, a closed canopy, and diverse tree recruitment. Photos by Andy Kulikowski.

So, how do we predict which sites will recover quickly and which ones need some help in the form of clearing pasture grasses and planting trees? If we could develop some rules of thumb it would help land managers to more efficiently allocate scarce restoration funds.

To answer this question, we drew on our long-term study on tropical forest restoration in southern Costa Rica. We have research plots at 13 different sites where we removed the land from agriculture and let the forest recover on its own. Each year we measure grass cover, tree canopy cover, and how many and what species of new tree seedling establish in the plots. We have also quantified the forest cover surrounding the plots, the nutrients in the soil, and how long cows had grazed the sites in the past.

We found that two easy-to-measure variables explained on average two-thirds of variation in forest recovery 7 years later; those were the amount of grass cover and tree canopy cover measured after only 1.5 years. Plots that had more canopy cover and lower grass cover early on had a closed tree canopy and lots of forest tree seedlings from many species after nearly a decade. We were surprised that the amount of surrounding forest cover and soil nutrients did not explain much of the variation in forest recovery.

Rules of thumb for predicting tropical forest regeneration on farmlands. Forests grow back quicker when there is not too much grass, a little bit of shade, and many tree seedlings already present. Illustrations by Michelle Pastor.

Of course, our results need to be tested in other recovering tropical forests. But, if they hold true, this is good news! It means that land owners and managers just need to wait a year or two and then measure the tree canopy and grass cover. If some trees have established and are starting to shade out the grasses, land managers can use the low cost method of leaving the site to recover naturally. If the site is mostly a monoculture of dense grass, then the site is a good candidate to plant native trees. Planting trees takes more resources since it is necessary to clear around the native tree seedlings for a couple of years until they grow taller than the grasses. At least now there are some general guidelines to help chose where to invest the extra effort.

CCSD scientist Leighton Reid and Lyon Arboretum director Rakan Zahawi write about giant fig tree cuttings: how to make them and why some grow better than others.

Choosing the right species to include in a restoration project is a hard choice, but in the economy of nature, some species earn a bigger ROI than others. For example, Pacific sea otters maintain kelp forests by eating sea urchins, and wolves in Yellowstone National Park allow aspen groves to regenerate by scaring away tree-munching elk. These vital creatures are called “keystone species” because they hold ecosystems together, much like the keystone in an arch.

Plants can be keystone species too. Around the world there are about 800 species of fig trees, and they hold tropical forests together by providing food for a wide array of animals. On any given day, the busiest tree in a rain forest is likely to be a fig tree with fruits. Monkeys, birds, bats, and others gather at fig trees to eat, and in the process, they deposit seeds of other plant species that they have been carrying in their guts. This chain of events, repeated day after day, often turns the area beneath a fig tree into a hotspot of plant diversity.

A few years ago, we had an idea to plant keystone fig trees in young forests in Costa Rica. We wanted the figs to grow as fast as they could, so instead of planting seedlings, we planted cuttings – big ones. With help from our local collaborator, Juan Abel Rosales, we cut dozens of twelve foot-long branches from eight species of fig trees. We stripped off all of their leaves to keep them from drying out, and then we planted our figs trees in shallow holes.

Rakan Zahawi (delighted!) poses with a three year-old fig stake.

To our delight, many of the fig trees grew!

The ones that did the best came from a special group, the subgenus Urostigma. Many figs in this group have a unique life strategy. They begin their lives in the top of a tree when their tiny seeds are deposited on a branch by a bird or some other animal. As they grow in the treetop, they send long roots down to the ground, and these roots harden and fuse together, forming a lattice-like trunk. Over time, these figs kill their host trees by taking most of the water, nutrients, and light. They also keep the host tree from growing outwards, giving them the nickname “strangler figs”. Maybe the ability to transform a flimsy, dangling root into a solid trunk is related to these figs being able to grow from cuttings.

To find out how well our planted fig cuttings might survive over the long-term, we also tracked down some fig cuttings that we had planted in 2004. We were happy to learn that out of the trees that survived for their first three years of life, all of them were still thriving a decade later.

Full disclosure: planting large cuttings is not a new idea. Farmers in many parts of the tropics plant trees this way to create ‘living fences’ – with all of the normal fixings like gates and barbed wire, but with a row of living trees instead of dead posts. The advantages for farmers are many – their fences don’t rot and fall apart (that happens quickly in the tropics); the trees provide shade for cattle; they have a constant source of new fence posts (by cutting off a limb); and in some cases they can feed the young shoots to livestock.

Big cuttings have big benefits for restoration too. Not only are planted trees already several feet tall, you also get to skip the pricey nursery phase, and, most excitingly, cuttings have a tendency to fruit quickly.

Some of our young fig trees are now making fruit, but we will have to wait a bit longer to see whether they start attracting more big animals and whether those animals carry more tree seeds into our young forests. For now, we can say that others who are interested in growing keystone figs for forest restoration may have the best luck by working with the stranglers.

How to grow an instant fig tree. (A) Remove a long, thin branch segment from an adult tree. The red arrow shows a cut branch. (B) Strip the cuttings of their leaves to keep them from drying out, then carefully transport cuttings so as not to damage cortical tissue. Here, cuttings are padded by a foam mattress. (C) Remove the bark from a ring on the cutting to promote root growth. Here, a ring is being cut about 20 cm (8 in) above the base so that it will be just below the soil surface when planted. (D) Dig a shallow hole and plant the cutting. Be sure that the cutting is firmly planted to prevent it from toppling, but take care not to compact the soil too much around its roots. Photos by Rakan Zahawi.

How should we restore forests in places where they have been lost? This is one of the main questions that we study in the CCSD, so we were surprised last year when a big synthesis paper that compiled data from many earlier studies said that, when it comes to restoration, doing nothing was the same as doing something.

That’s only a slight exaggeration. The paper, by Renato Crouzeilles and several other scientists, said that letting a forest regrow on its own (that is, natural regeneration) was usually more successful than planting trees (that is, active restoration). Their conclusion was based on comparing many studies done throughout the world’s tropical forest regions.

Apples to oranges

The problem with this paper (and several like it) was that the set of studies looking at natural regeneration were not really the same as the set of studies looking at tree planting. The natural regeneration studies focused on forests that already existed, while the tree planting studies focused on a wider range of sites, many of which started with no forest. In other words, the natural regeneration studies had already been filtered to exclude places with a weak ability to grow forests.

Comparing tree planting studies to pre-existing forests is like comparing apples and oranges. Photo by Michael Johnson CC BY 2.0.

To understand the problem, it is helpful to look back at the history of tropical forest restoration research. For many years, scientists who wanted to know about how forests recover after a disturbance (like a hurricane or logging) would go out and find several forests that had been recovering for different amounts of time. If you take forests that are 5, 10, and 20 years old, you can try to compare them to each other in order to see how a forest might change over 20 years. In contrast, tree planting studies usually start with a piece of land that has no trees on it. Scientists who want to know how trees grow on this land will plant some and then observe their survival and growth over time. These trees may or may not create a forest there, as the land can vary in quality.

So where does that bring us with respect to this study? If you compare a forest that already exists with another potential forest where planted trees may or may not survive and grow well, it’s a safe bet that the pre-existing forest will have taller trees. It has a head start over the planted forests, and we argue in our paper that the comparison is not a fair one.

This means that letting forest regrow on its own is not always a better option than planting trees. In fact, there are many places – like overgrazed pastures, mine sites, and other heavily degraded lands – where forests have been cleared and most likely will not be able to grow back on their own.

Comparison of natural regeneration (foreground) and active tree planting (background) to restore a cattle pasture in southern Costa Rica. Tree seedlings planted on the hillside are just visible in the 2005 image. The yellow circle indicates a person for scale. After nine years, active tree planting had produced a forest, whereas natural regeneration was stalled. Overgrown pasture grasses covered the ground. Natural regeneration is highly variable, so this example is not representative of all situations. Photos courtesy of Karen Holl.

Same team!

While we were not convinced by studies that said that natural regeneration is better than tree planting, we also don’t want to take any options off the table. Natural regeneration and tree planting are not mutually exclusive – in fact, they are highly complementary. Our practical advice is that if you want to get forest back, the best option is to see if natural regeneration can do the trick before you invest in tree planting. Or better yet, set up a paired experiment comparing the two strategies at the same site.

The Brazilian Atlantic Forest is a global biodiversity hotspot. This designation denotes two things. First, the Atlantic Forest is exceptionally and uniquely biodiverse. Second, the biodiversity of the Atlantic Forest is exceptionally threatened. This once-vast biome historically stretched from northern Argentina to Brazil’s eastern tip in Rio Grande do Norte, but it is now reduced to about 12% of its original size, and most of what remains exists as small, isolated fragments.

During the past decade, a major, multilateral effort has been undertaken to staunch biodiversity loss by doubling the size of the Atlantic Forest through ecological restoration. The Atlantic Forest Restoration Pact is composed of more than 270 private companies, governments, NGOs, and research organizations. It aims to restore 15 million hectares of Atlantic Forest by 2050.

The Atlantic Forest biome: a global biodiversity hotspot and the site of the most ambitious tropical forest restoration project on the planet. Map imagery from NASA via Wikimedia Commons.

Atlantic Forest restoration projects are characteristically thorough and well-documented. For example, they often include high diversity plantings more than 80 tree species. Yet until recently there had never been a systematic study to evaluate how well these restoration plantings represented the Atlantic Forest biodiversity they aimed to protect.

Dr. Pedro Brancalion is a professor at the University of São Paulo’s agricultural school in Piracicaba, Brazil, where he co-directs the Tropical Silviculture Lab. Five years ago, he approached me at a meeting of the Society for Ecological Restoration in Madison, Wisconsin, and over a beer he told me about a dataset that he thought could shed light on the how well Atlantic Forest restoration projects were conserving tree biodiversity. The dataset consisted of seedling donation records from the NGO SOS Mata Atlântica. Between 2002 and 2015, the NGO donated more than 14 million tree seedlings to 961 restoration projects. By comparing the species composition in these records to tree species living in mature forests, we could see what elements of biodiversity might be missing and how this could be affecting carbon stocking – an important factor in mitigating global climate change.

Even in high diversity plantings, many of the most threatened tree species were not included.

Last month, Pedro and our collaborative team published a paper in Conservation Letters describing our results. We found that restoration projects in the Atlantic Forest biome had included 416 tree species out of the >2,500 tree species known from mature and old-growth forest fragments. This is an impressive figure, but the team discovered that it reflects a highly biased subsample of the Atlantic Forest tree flora. The most under-represented species were those with large seeds that are dispersed by animals. Animal-dispersed trees make up as much as 89% of tree species in some parts of the Atlantic Forest and include some of the most threatened species.

The reason that large-seeded, animal-dispersed species are being used less often was probably related to the cost and challenges of collecting and growing seeds. Large-seeded, animal-dispersed trees are more expensive to purchase from nurseries than small-seeded or wind-dispersed species. Because they are energetically expensive to produce and are contained within large fruits, trees tend to produce large seeds in relatively low quantities, with just one or a few seeds per fruit. They are generally found in remote forest areas, and seed collectors have to compete for them with seed-eating animals, like peccaries and agoutis. Once large seeds are collected, they also take up considerably more space in storage and production facilities.

The absence of large-seeded, animal-dispersed tree species in restoration plantings has important implications for biodiversity conservation. First, fewer large-seeded trees means less food for large birds, some of which eat mainly large fruits. Second, these species are sometimes overharvested for timber and have difficulty recolonizing forests from which they have been removed. So the fact that large-seeded, animal-dispersed trees are under-represented in restoration projects means that even if the ambitious restoration goals of the Atlantic Forest Restoration Pact are met, the increase in forest cover may not improve dispersal between fragmented populations of the most vulnerable species.

Large-seeded tree species also tend to store carbon more densely than small-seeded species. This tendency is related to large-seeded species growing slowly in the shady understory of the Atlantic Forest and their gradual formation of dense wood, which is rich in carbon. We simulated potential carbon stocking in restored forests and compared it to mature forests, and our results showed that under-representation of large-seeded, animal-dispersed trees could cause a 2.8-10.6% reduction in carbon storage. Based on the current price of carbon, this loss could represent $17-63 USD per hectare in lost carbon credits.

Many Atlantic Forest restoration projects are quite isolated. A large seed would have a hard time reaching sites like this forest in a sugarcane matrix. Photo by Pedro Brancalion.

Reduced capacity for biodiversity conservation and carbon stocking sounds like bad news, and indeed it is not ideal. However, restoration ecology moves forward by identifying problems and seeking scientifically-based solutions to overcome them. Knowing that large-seeded, animal-dispersed trees are under-represented in restoration plantings means that we can turn our attention to innovative solutions.

For example, new policies could help bridge the gap between Brazil’s exceptional tree biodiversity and the relative paucity of species being used for ecological restoration. One way this could happen would be for the Brazilian government to subsidize the cost of producing large-seeded, animal-dispersed tree seedlings. This could be done through financial incentives or potentially by opening some forest reserves for seed harvesting, to make it easier for collectors to acquire these species. Facilitating uptake by reducing costs would be a carrot. A stick could be to legally mandate some representation of these species in future restoration plantings.

Market solutions may also exist. Based on our calculations, adding more large-seeded, animal-dispersed species to restoration plantings would increase carbon storage and carbon credits, offsetting the cost of the expensive seedlings and creating a net gain of $3-32 USD per hectare.

Historically, intensive agriculture in the Brazilian Atlantic Forest has caused large-scale deforestation of this biome. However, new legal requirements, land exhaustion, and the shifting priorities of farmers have recently allowed forests to regenerate on some formerly farmed lands. Given the unique nature of the Atlantic Forest, its high species endemism, and its potential for providing ecosystem services, the Tropical Silviculture Lab (LASTROP) at the University of São Paulo, coordinated by Prof. Pedro Brancalion and its partners, initiated a project in 2014 to better understand the structure and composition of these new forests.

However, forests aren’t made solely of trees. Among the plant components of a forest, there are others life forms such as epiphytes, lianas, and herbs that contribute to biodiversity and provide food, water, and shelter for many animal species. Epiphytes are plants that use other plants as support. Due to their sensitivity to environmental changes, epiphytes can be used as bioindicators. Therefore, we asked how these plants are doing in these young regenerating forests. And what landscape and local attributes facilitate or hinder their recolonization?

To try to answer these questions, we are studying the epiphyte communities in 40 second-growth forests (i.e., forests that were once completely cut down). We are considering three landscape drivers (distance from watercourses, distance from forest edge and forest cover in a 1-km buffer around the remnant) and four local drivers (previous land use, forest age, liana abundance, and tree basal area). We expect that forests close to watercourses would provide the moisture required by epiphytes. We expect to find more epiphytes further from the forest edge since forest cover in more conserved forests may limit their establishment. Since our forests regenerated from abandoned eucalyptus plantation and pastures, we want to check if the non-native eucalyptus could act as a filter preventing epiphytes recolonization. We also expect that older forests and forests with more basal area could house more epiphytes than young forests. Finally, field observations made us wonder if lianas could compete with epiphytes by occupying the same niche.

Of the more than 6,000 phorophytes (trees that could support epiphytes) sampled in these 40 forests we found 398 epiphytes belonging to 21 morphospecies distributed in 4 families (Araceae – 1 species, Bromeliaceae – 14 species, Cactaceae – 5 species, Orchidaceae – 5 species). Only three species, Tillandsia pohliana, Tillandsia tricholepis, and Ionopsis utricularioides, represented more than half (59.5%) of all epiphytes found in second-growth forests. The genus Tillandsia was expected to be abundant in these young forests, since these are disturbance-adapted species that can even be found growing on power lines in cities.

Epiphytes of the genus Tillandsia (Bromeliaceae) are often found in extreme microenvironments in urban areas (Photos by A. Mendes, 2017).

Our analysis is in progress, but our preliminary observations suggest that forests closer to watercourses and closer to forest edge are more likely to have epiphyte recolonization than forests far from edge and watercourses. Forests regenerated on pastures have more epiphytes than those on abandoned eucalyptus plantation. Our dataset will soon be upgraded with new forest types: conserved and disturbed old-growth forests, and mixed tree plantings for forest restoration, totaling approximately 70 forests with epiphyte samples.

With this research, we hope to find out the local and landscape factors that contribute to epiphyte recolonization in second-growth forests. In practice, this will allow us to locate sites with limited potential for spontaneous colonization of this life form to take actions that promote colonization and establishment, such as introducing individuals. Finally, by identifying epiphytes species that are more sensitive to disturbance, we can focus our reintroduction interventions.